168 The Vertebrate Organ Systems 



supply the esophagus, and the phrenic artery suppHes the diaphragm. 

 In the abdominal cavity, the first vessel to arise is the coeliac which 

 supplies the liver, spleen, and stomach. Another, the superior mesen- 

 teric, breaks into many branches to supply the upper part of the di- 

 gestive tract, while more posterior, the inferior mesenteric supplies the 

 lower part of the digestive tract. Between these two vessels, others 

 arise, including the large renal arteries which supply the kidneys, the 

 smaller adrenals for the adrenal glands, and the gonadals to the ovaries 

 or testes. The large iliolmnbar arteries supply the muscles of the lum- 

 bar region. All these arteries branch into arterioles which divide and 

 subdivide to become capillaries. The actual exchange of oxygen, car- 

 bon dioxide, foods, etc., probably occurs through the thin walls of 

 the capillaries. Gradually the (;apillaries unite to form the venuoles 

 which further unite to form the veins. 



The Systemic Venous System. — In man and many other mam- 

 mals (Fig. 49), the blood from the anterior part of the body is re- 

 turned to the right auricle of the heart by the precaval vein. This is 

 formed in the neck region by the junction of the right and left in- 

 nominates. Each of these latter is formed by the union of the several 

 jugulars and the subclavian. The blood from the chest and arms enters 

 the paired subclavians through several vessels. An unpaired vein, the 

 asygos, picks up the blood from the intercostal veins and empties it 

 into the precaval. 



The blood from the region posterior to the heart enters through 

 the postcaval vein. This median vein is in the dorsal portion of the 

 body cavity, penetrating the diaphragm in the same region as the aorta. 

 Along its way, it picks up tributary veins which drain blood from the 

 various organs. Some of these veins include the phrenic vein from 

 the diaphragm, the hepatic vein from the liver, the renal veins from the 

 kidney, the gonadal veins from the gonads, and the iliolnmbars 

 from the lumbar muscles. At its posterior end, the postcaval is formed 

 by the union of the two common iliac veins which drain the legs. In 

 some animals, the prominent caudal vein is also present in this region. 



Hepatic Portal System. — One division of the systemic veins drains 

 the digestive tract and associated organs. This is the hepatic portal 

 system, and it is the only division of the circulatory system that begins 

 and ends in capillaries. These capillaries begin in the walls of the di- 

 gestive tract. They unite to form several diliferent veins, including the 

 superior and inferior mesenteries, the gastroepiploic, the splenic, and 



